Bluetooth: Controller: Fix CIS failed to be established

Fix CIS create implementation to consider CIG events with
laziness before the new CIS in an already active CIG is
made active.

Previous laziness value of the CIG events is determined and
at the CIG event where the new CIS event_count is calculated
the lazy_active value determined is decremented from the
total lazy value.

Without this fix, event_count of the new active CIS events
where incorrect, causing CIS create resulting in fail to be
established.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>